Optical amplifier arrangement for a solid state laser

ABSTRACT

An optical amplifier arrangement, including an amplifying medium, which exhibits an approximately rectangular cross section with a long edge and a short edge, as well as at least two highly reflecting mirrors, between which the amplifying medium is disposed, whereby the long or the short edge of the cross section is along the X axis or the Y axis; the Z axis is the optical axis; and the X, Y and Z axis form a rectangular coordinate system. The mirrors are designed and arranged in such a manner that one beam, which is to be amplified and beamed in by an oscillator, passes repeatedly through the amplifying medium in the XZ plane and is amplified; and the size of the beam to be amplified in the X direction becomes larger after each passage.

BACKGROUND OF THE INVENTION

The present invention relates to an optical amplifier apparatus that issuited, in particular, for amplifying at a high amplification factor alaser beam emitted from a laser source or from an oscillator.

The achievable laser output power, in particular of solid-state lasers,is generally limited with regard to high beam quality by the thermallens effect of the amplifying medium. An oscillator-amplifier apparatushas been in use to date to achieve high laser performance accompanied byhigh beam quality. The oscillator is arranged in such a way that itemits a laser beam of high quality at relatively low power.Subsequently, the laser beam, which is emitted by the oscillator, isirradiated into the amplifier (connected in series) and amplified,resulting in high beam power while maintaining the beam quality.

An apparatus of this type, including an amplifier connected in series,is shown e.g. in FIG. 6, wherein the reference symbol 1 denotes anoptical amplifier, reference symbol 13 denotes an oscillator andreference symbol 14 denotes the optical components that serve to realizethe optical imaging.

The apparatus represented in FIG. 6 is a laser apparatus in which boththe oscillator and the amplifier have a rod-shaped solid-state medium.Based on the available amplification, which is limited ultimatelybecause of increased spontaneous emission and parasitic oscillation, thetypical amplification factor for an amplifier of this kind isapproximately 1.2 to 3 per run.

In weak oscillators, this amplification factor is often too minimal toachieve an efficient utilization of the amplification medium.

One possibility for solving this problem is a regenerative amplifierapparatus where the amplification medium is integrated in a resonatorwith a Pockels cell and a polarizer (regenerative resonator). The laserbeam that is to be amplified is injected in the regenerative resonatorthrough the Pockels cell and the polarizer. After several runs throughthe amplifier medium, the laser beam is amplified several times, and inthe end it is coupled out of the resonator via the Pockels cell and thepolarizer. Even though this method produces an efficient amplification,the described realization is very complex and is applicable only to alimited degree for laser pulses that are shorter than several 10 ps.

To further increase the laser performance several amplification stepscan be used. Multi-step amplifier apparatuses of this kind always comeat a high cost. Moreover, they are very voluminous, need much space andare not very reliable.

To improve the amplification factor, the U.S. Pat. No. 4,703,491discloses a laser apparatus with a partially permeable feed-out mirrorand a fully reflective folding mirror on the one side of the activelaser medium and another fully reflective folding mirror on the otherside of the active laser medium. The optical system is arranged in sucha way that a laser beam passes through an active laser medium severaltimes resulting in a long, effective resonator length. In thisapparatus, the beam returns on to itself after multiple folding ormulti-pass.

Furthermore, German patent document DE-A-196 09 851 describes amicro-strip laser with an almost fully reflective end mirror and apartially reflecting feed-out mirror that are arranged, respectively, ondifferent sides of the active laser medium, and two folding mirrors thatare inclined in the direction of the face sides of the resonator on bothsides of the laser medium. This apparatus realizes a multi-passresonator. As shown in particular in FIG. 3, one of the folding mirrorshas a mirror surface with a concave curvature, which expands thecross-section of the laser beam in one pass-through direction. In thereverse pass-through direction, however, the cross-section of the laserbeam is once again reduced. This leads to increased stress being placedon the optical components within the resonator. In fact, when passingfrom the feed-out mirror to the fully reflective end mirror, the powerdensity drastically increases because the cross-section of the beam isbecoming smaller. This not beneficial for the effective utilization ofthe amplifier medium and in terms of the stress on the opticalcomponents.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an opticalamplifier apparatus that eliminates the disadvantages of the state ofthe art.

This and other objects and advantages are achieved by the opticalamplifier according to the invention, which includes an amplifier mediumthat has approximately a rectangular cross-section, with a long edge anda short edge, and at least two highly reflective mirrors. The amplifyingmedium is arranged between the latter mirrors, with the long edge orshort edge of the cross-section disposed along the x-axis or the y-axis,and the z-axis being the optical axis. (The x-, y- and z-axes constitutea rectangular system of coordinates.) The mirrors are designed andarranged in such a manner that a beam injected by an oscillator into thexz-plane, which is to be amplified, passes through the amplifying mediumseveral times and is amplified, while the dimension of the beam, that isto be amplified, expands in the x-direction after each pass-through.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a cross-section in the xz-plane of a first embodiment of thepresent invention;

FIG. 1b is a cross-section in the yz-plane of a first embodiment of thepresent invention;

FIG. 1c is a cross-section of the amplification medium in the xy-plane;

FIG. 2 is the schematic expansion of the beam that is to be amplified inaccordance with a second embodiment of the invention;

FIG. 3a shows a cross-section in the xz-plane of a third embodiment ofthe present invention;

FIG. 3b shows a cross-section in the yz-plane of a third embodiment ofthe present invention;

FIG. 4 shows a solid-state amplification medium that has a doped mediumarea and two undoped edge areas in the y-direction;

FIG. 5 shows an amplifier apparatus in accordance with another embodiedexample of the present invention; and

FIG. 6 shows a conventional amplifier apparatus.

DETAILED DESCRIPTION OF THE INVENTION

The elements of the optical amplifier apparatus according to the presentinvention are a rod-shaped amplification medium and two speciallydesigned and arranged highly reflective mirrors, the amplificationmedium being arranged between the latter. The advantages of therod-shaped amplification medium reside in its quasi one-dimensional heatconduction and an one-dimensional lens effect representing minimal lossdue to depolarization for a solid-state medium.

In FIGS. 1a-1 c, the amplification medium with an approximatelyrectangular cross-section is arranged between at least two highlyreflective mirrors 2, 3. (A rectangular system of coordinates isintroduced to simplify the representation.) The x-axis or y-axis isparallel to the long edge or short edge of the cross-section, and thez-axis is parallel to the optical axis. An oscillator (not shown) emitsa laser beam with high beam quality and relatively minimal power. Thebeam 4 from the oscillator (input beam) that is to be amplified isinjected essentially parallel in relation to the optical axis into theamplification medium 1. If the two mirrors are suitably dimensioned, thebeam passes multiple times back and forth, principally through thexz-plane, between the mirrors and through the amplification medium 1.This arrangement ensures that, on the one hand, the high beam quality ofthe amplified beam is not compromised and, on the other hand, that ahigh amplification factor is realized.

In accordance with the present invention, the optical amplificationapparatus, preferably the two mirrors, is designed in such a way thatthe dimension of the output beam becomes larger in the x-direction thanthe dimension of the input beam. During amplification of the beam thatis passing through, this will allow for achieving an approximately evenpower density.

The above is advantageous with regard to the saturation behavior of theamplification and for reducing the intensity with regard to the opticalcomponents, in particular the amplification medium.

In fact, efficient operation of the amplifier presupposes that the laserintensity is comparable with the saturation intensity of the lasermedium. This means that if the laser performance increases, thecross-section of the laser beam must be expanded correspondingly. Themost effective utilization of the amplification is accomplished when thedimension of the beam expands in the x-direction after eachpass-through, preferably at a constant factor M.

The factor M is a function of the small signal gain and of the intensityof the input beam in relation to the saturation intensity. For theamplification of the cw- and/or qcw-lasers the factor M is representedby:

M≈1+(l _(s) /l _(in))·gol

wherein l_(in) denotes the intensity of the input beam, l_(s) thesaturation intensity of the amplification medium and gol the total smallsignal gain per run.

For a pulsed laser the optimum factor M is represented as:

M≈1+(E _(s) /E _(in))·gol

wherein E_(in) denotes the energy flow density of the input beam, E_(s)the saturation energy flow density of the amplification medium and goethe total small signal gain per run.

A particularly simple amplification apparatus is depicted in FIG. 2. Itis realized with two planar mirrors, which are arranged at an angle βrelative to each other that is approximately equal to the fulldivergence angle α of the input beam. For efficient amplification, theangle α is defined principally by the small signal gain, the saturationintensity, the power of the input beam and the distance between the twomirrors.

Another preferred embodied example is shown in FIG. 3a. In this embodiedexample the two mirrors 2, 3 are designed and arranged in such a waythat they form a type of hybrid resonator that is unstable in thexz-plane and stable in the yz-plane. The input beam 4 is injected in theresonator off axis.

To stabilize the optical resonator in the yz-plane it is possible to useone or several cylindrical lens(es).

In this embodiment the two mirrors 2, 3 are realized cylindrically withcurvatures in the xy-plane. It should be noted that this embodiment canalso be realized in a more general optical amplifier apparatus with thedimension of the beam that is to be amplified expanding in thex-direction; but it does not expand after each pass-through. Theresonator is stable in the yz-plane due to the thermal lens effect.

It is advantageous that this embodiment needs fewer optical components,which reduces the adjustment effort for the optical amplifier apparatus.

Furthermore, it is advantageous if the amplification medium has awave-guiding function at least in the y-direction.

In particular, simple beam paths can be realized in a confocal unstableresonator. As represented in FIG. 3a, in this resonator the focal pointsof the two mirrors 2, 3 are coincident at a point F. To avoid anypossible destruction of optical components, it is advantageous to placethe joint focus at a location outside of the resonator.

A constant dimension of the beam within the amplification medium 1 inthe y-direction is advantageous with regard to efficiency and beamquality. This can be achieved if the radius and position of the beamwaist of the input beam 4 is adjusted to the transversal modes of thestable resonator. If both mirrors are straight in the y-direction, thebeam waist of the input beam 4 is to be at the position of the mirror 2that points toward the oscillator. Taking into consideration the lenseffect of the amplification medium 1, the radius of the beam waist inthe yz-plane is defined in such a way that, subsequent to the passagethrough the amplification medium in the yz-plane, another beam waist iscreated at the position of the mirror 3 that points away from theoscillator, as illustrated in FIG. 3b.

By placing the beam waist of the input beam in the xz-plane on the jointfocus F of the two mirrors 2, 3 it is possible to realize a kind of lensconduction with a constant expansion in the xz-plane.

In total, amplification factors of between 10 and 1000 are achieved withthe optical amplification apparatus according to the invention.

All laser-suitable media such as e.g. gas, excimer, semiconductor orsolid-state media in which the population inversion takes place,respectively, by way of gas discharge, current injection or opticalpumping, are suitable for use as the amplification media.

If a solid-state medium is used for the optical amplifier apparatusaccording to the invention, it is advantageous, in terms of its thermalbehavior, to use a sandwich structure in the y-direction and/or in thez-direction. And the solid-state has in the y-direction and/or in thez-direction at least one doped medium area 7 and two undoped edge areas6, 8, as shown in FIG. 4.

The solid-state medium can then be pumped optically, for example, usinga diode laser radiation. When pumping, it is of fundamental importanceto realize the pumping apparatus in such a way that a homogeneouspumping performance is realized in the xy-plane.

FIG. 5 shows another embodiment of the present invention, in which thisa solid-state amplification medium is pumped through a diode laserapparatus 9. In the interest of simplicity, the highly reflectivemirrors 2, 3 and the baser beam 4 that is emitted by the oscillator andis to be amplified, are not shown in FIG. 5. But they can be arranged asshown in FIGS. 1a, 2 and 3 a.

As seen in FIG. 5, the pump radiation that is emitted from thediode-laser apparatus 9 is coupled in the amplification medium 1 atleast through one of the end surfaces and principally parallel inrelation to the z-direction. The diode-laser apparatus 9 is realized insuch a way that a flat pumped channel with a rectangular cross-sectionresults in the amplification medium 1, whose dimension in they-direction is smaller than that of the amplification medium. This pumpapparatus is advantageous due to its improved thermal properties, and itoffers fewer opportunities for parasitic oscillation.

If a solid-state medium is used as amplification medium 1, the contactcooling 12 effected through the two large surfaces that are parallel inrelation to the xz-plane, ensures that the dissipation heat iseffectively removed, as seen in FIG. 5.

A homogenization of the diode-laser pump radiation can be realized byfocusing it into a planar wave-guide, with the exit end of thewave-guide being imaged into the amplification medium by way of imagingoptics. The cross-section of the planar wave-guide is oriented in such away that its large dimension is along the x-direction.

In another aspect, the present invention relates to an optical amplifierapparatus including an amplification medium that has an approximatelyrectangular cross-section with a long edge and a short edge as well asat least two highly reflective mirrors, the amplification medium beingarranged between the latter. The long edge or the short edge of thecross-section is disposed along the x-axis or the y-axis, with thez-axis being the optical axis. (The x-, y- and z-axes constitute arectangular system of coordinates). The mirrors are designed andarranged in such a way that a beam, which is to be amplified, is emittedby an oscillator and passes through the amplification medium in thexz-plane several times and becomes amplified. A hybrid resonator isformed that is unstable in the x-direction and stable in the y-directionand for which the input beam is injected in the amplification medium offaxis, while the two highly reflective mirrors are dimensionedaccordingly so that the resonator in the yz-plane is stable with theinvolvement of the thermal lens effect in that plane.

In still another aspect, the present invention relates to a laseroscillator with an amplification medium that has an approximatelyrectangular cross-section with a long edge and a short edge as well asat least two highly reflective mirrors, the amplification medium beingarranged between the latter. The long edge or the short edge of thecross-section is disposed along the x-axis or the y-axis, with thez-axis being the optical axis. (The, and the x-, y- and z-axesconstitute a rectangular system of coordinates.) The mirrors aredesigned and arranged in such a way that a multi-pass resonator iscreated and a hybrid resonator is formed, which is unstable in thex-direction and stable in the y-direction; and the two highly reflectivemirrors are dimensioned accordingly so that the resonator in theyz-plane is stable with the involvement of the thermal lens effect inthat plane. The resonator is designed in such a way that a laseroscillation takes place. More specifically, the connecting line of thecurvature centers of the two mirrors is within the amplification zonethat is enclosed by the reflecting mirror ranges.

In the described optical amplifier apparatus or the laser oscillator,the resonator is preferably stable in the yz-plane exclusively due tothe thermal lens effect.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. Optical amplifier apparatus comprising: an amplification medium that has an approximately rectangular cross-section with a long edge and a short edge, at least two highly reflective mirrors, wherein the amplification medium is arranged between the at least two highly reflective mirrors, wherein the long edge or the short edge of the cross-section is along the x-axis or the y-axis, and the z-axis is the optical axis, and wherein the x-, y- and z-axes constitute a rectangular system of coordinates, wherein the mirrors are designed and arranged in such a way that a beam that is to be amplified, emitted in by an oscillator, passes through the amplification medium in the xy-plane several times and becomes amplified, wherein the dimension of the beam expands in the x-direction after each passage through, and wherein the resonator is stable in the yz-plane due to a cylindrical lens effect.
 2. The optical amplifier apparatus as claimed in claim 1, wherein the dimension of the input beam expands by a constant factor in the x-direction after each passage through.
 3. The optical amplifier apparatus as claimed in claim 1, wherein: the at least two mirrors are planar mirrors; and the mirrors are arranged at an angle β relative to each other which is approximately equal to a full divergence angle α of the input beam.
 4. The optical amplifier apparatus as claimed in claim 1, wherein the at least two mirrors are configured and disposed such as to form a hybrid resonator that is unstable in the x-direction and stable in the y-direction; and the input beam is injected in the amplification medium off axis.
 5. The optical amplifier apparatus as claimed in claim 1, wherein the two mirrors are cylindrical mirrors with curvatures in the xz-plane; and the resonator is stable in the yz-plane due to a thermal lens effect in this plane.
 6. The optical amplifier apparatus as claimed in claim 1, wherein the at least two mirrors have a joint focus F in the xz-plane and form a confocal unstable resonator.
 7. The optical amplifier apparatus as claimed in claim 1, wherein the joint focus is located outside of the resonator.
 8. The optical amplifier apparatus as claimed in claim 1, further comprising at least one lens arranged inside the resonator for beam formation or beam path and mode volume configuration.
 9. The optical amplifier apparatus as claimed in claim 1, wherein at least one of the lenses arranged in the resonator is cylindrical in the yz-plane.
 10. The optical amplifier apparatus as claimed in claim 1, further comprising means for transforming the input beam before injection in the amplification medium such that a characteristic curve thereof corresponds to a transversal mode of the stable resonator in the yz-plane.
 11. The optical amplifier apparatus as claimed in claim 1, wherein a waist of the input beam is in the joint focus of the two mirrors.
 12. The optical amplifier apparatus as claimed in claim 1, wherein the amplification medium is a gas medium.
 13. The optical amplifier apparatus as claimed in claim 1, wherein the amplification medium is a solid-state medium.
 14. The optical amplifier apparatus as claimed in claim 1, wherein the solid-state medium has a sandwich structure with at least one doped medium area and two undoped edge areas.
 15. The optical amplifier apparatus as claimed in claim 1, wherein the solid-state medium is optically pumped.
 16. The optical amplifier apparatus as claimed in claim 1, wherein the solid-state medium is pumped using diode lasers.
 17. The optical amplifier apparatus as claimed in claim 1, wherein the solid-state medium is pumped at least through one end surface, substantially parallel to the z-axis.
 18. The optical amplifier apparatus as claimed in claim 1, wherein the solid-state medium is pumped at least through one side surface, substantially perpendicular to the z-axis.
 19. The optical amplifier apparatus as claimed in claim 1, wherein the solid-state medium has a flat, pumped channel with a rectangular cross-section whose dimension in the y-direction is smaller than a dimension of the doped solid-state medium.
 20. The optical amplifier apparatus as claimed in claim 1, further comprising at least one optical pump source is arranged such that an approximately homogeneous pump power distribution in present in the xy-plane.
 21. The optical amplifier apparatus as claimed in claim 1 with a planar wave-guide for homogenizing pump radiation with regard to the intensity in the xy-plane.
 22. Optical amplifier apparatus comprising: an amplification medium that has an approximately rectangular cross-section with a long edge and a short edge, at least two highly reflective mirrors, wherein the amplification medium is arranged between the at least two highly reflective mirrors, wherein the long edge or the short edge of the cross-section is along the x-axis or the y-axis, and the z-axis is the optical axis, and wherein the x-, y- and z-axes constitute a rectangular system of coordinates, wherein the mirrors are designed and arranged in such a way that a beam that is to be amplified, emitted in by an oscillator, passes through the amplification medium in the xy-plane several times and becomes amplified, wherein the dimension of the beam expands in the x-direction after each passage through, and wherein the resonator is stable in the yz-plane as a result of a thermal lens effect in this plane.
 23. The optical amplifier apparatus as claimed in claim 22, wherein the dimension of the input beam expands by a constant factor in the x-direction after each passage through.
 24. The optical amplifier apparatus as claimed in claim 22, wherein the at least two mirrors are planar mirrors; and the mirrors are arranged at an angle β relative to each other which is approximately equal to a full divergence angle α of the input beam.
 25. The optical amplifier apparatus as claimed in claim 22, wherein the at least two mirrors are configured and disposed such as to form a hybrid resonator that is unstable in the x-direction and stable in the y-direction; and the input beam is injected in the amplification medium off axis.
 26. The optical amplifier apparatus as claimed in claim 22, wherein: the two mirrors are cylindrical mirrors with curvatures in the xz-plane; and the resonator is stable in the yz-plane due to a thermal lens effect in this plane.
 27. The optical amplifier apparatus as claimed in claim 22, wherein the at least two mirrors have a joint focus F in the xz-plane and form a confocal unstable resonator.
 28. The optical amplifier apparatus as claimed in claim 22, wherein the joint focus is located outside of the resonator.
 29. The optical amplifier apparatus as claimed in claim 22, further comprising at least one lens arranged inside the resonator for beam formation or beam path and mode volume configuration.
 30. The optical amplifier apparatus as claimed in claim 22, wherein at least one of the lenses arranged in the resonator is cylindrical in the yz-plane.
 31. The optical amplifier apparatus as claimed in claim 22, further comprising means for transforming the input beam before injection in the amplification medium such that a characteristic curve thereof corresponds to a transversal mode of the stable-resonator in the yz-plane.
 32. The optical amplifier apparatus as claimed in claim 22, wherein a waist of the input beam is in the joint focus of the two mirrors.
 33. The optical amplifier apparatus as claimed in claim 22, wherein the amplification medium is a gas medium.
 34. The optical amplifier apparatus as claimed in claim 22, wherein the amplification medium is a solid-state medium.
 35. The optical amplifier apparatus as claimed in claim 22, wherein the solid-state medium has a sandwich structure with at least one doped medium area and two undoped edge areas.
 36. The optical amplifier apparatus as claimed in claim 22, wherein the solid-state medium is optically pumped.
 37. The optical amplifier apparatus as claimed in claim 22, wherein the solid-state medium is pumped using diode lasers.
 38. The optical amplifier apparatus as claimed in claim 22, wherein the solid-state medium is pumped at least through one end surface, substantially parallel to the z-axis.
 39. The optical amplifier apparatus as claimed in claim 22, wherein the solid-state medium is pumped at least through one side surface, substantially perpendicular to the z-axis.
 40. The optical amplifier apparatus as claimed in claim 22, wherein the solid-state medium has a flat, pumped channel with a rectangular cross-section whose dimension in the y-direction is smaller than a dimension of the doped solid-state medium.
 41. The optical amplifier apparatus as claimed in claim 22, further comprising at least one optical pump source is arranged such that an approximately homogeneous pump power distribution in present in the xy-plane.
 42. The optical amplifier apparatus as claimed in claim 22 with a planar wave-guide for homogenizing pump radiation with regard to the intensity in the xy-plane. 